The present invention is directed to a semiconductor component, particularly to a semiconductor component with a high-efficiency barrier junction termination.
In general, a semiconductor component contains at least one active semiconductor area and a semiconductor region acting as either an n or p type drift region. Such a semiconductor component also includes two electrodes for applying an operating voltage to the drift region, as well as usually other semiconductor regions for forming component-specific semiconductor structures. In an on state of the component, the drift region carries the electric current of the charge carriers between the two electrodes. In the off state of the component, however, the drift region takes on a depletion region of a p-n junction formed with the drift region or of a metal-semiconductor barrier contact (Schottky contact) which is formed as a result of the operating voltages applied that are relatively high compared to those in the on state. The depletion region is often also called the space charge region or barrier layer. Distinction is made between unipolar and bipolar semiconductor areas. In unipolar active semiconductor areas, only one kind of charge carriers (electrons or holes) determines the operation, while in bipolar active semiconductor areas, both charge carrier types, electrons and holes, contribute to the operation.
In the off state, relatively strong electric fields are created on the surface of the component. Therefore it is important to ensure the stable transition of these electric surface fields into the medium surrounding the component with a maximum field intensity that is clearly below the average field intensity of the surrounding medium. The surrounding medium can be dielectric insulating and/or passivating layers, or a surrounding gas, usually air. The problem of excessively high field intensities on the surface of a component appears especially in the case of high off-state voltages, which occur in power electronics applications in which small dimensions with high field line curvatures or high doping of the semiconductor regions is present. To reduce the field intensity on the surface of the component, a device known as a junction termination is used. The junction termination is produced on the component surface and surrounding the active semiconductor area. In addition to electrically shielding the active semiconductor area outward, the junction termination also reduces the field line curvatures around the active semiconductor area in order to diminish excessively intense fields in the area close to the surface within the semiconductor component.
Different embodiments of junction terminations for p-n junctions of silicon-based power electronics semiconductor components are described in xe2x80x9cModern Power Devicesxe2x80x9d by B. J. Baliga, 1987, John Wiley and Sons (USA), pp. 79-129. Such p-n junctions are usually produced by diffusion of a dopant into the surface of a silicon layer acting as a drift region, with the diffused region being of the opposite conduction type compared to the silicon layer. Due to field line curvature, an extra-high intensity field is produced at the edge of the diffused region compared to the planar p-n junction, depending on the depth of this region.
In a first known embodiment, floating field rings can be provided as junction terminations, which are also produced through diffusion around the diffused region of the p-n junction of the silicon layer. These field rings are of the same conduction type as the diffused layer of the p-n junction and are insulated from the diffused region and from one another by the silicon layer doped for the opposite conduction type. One or more field rings can be provided. A second method for obtaining a junction termination consists of removing material, and thus charges, from around the edge of the p-n junction by mechanical abrasion or etching (xe2x80x9cbeveled-edge terminationxe2x80x9d or xe2x80x9cetch contour terminationxe2x80x9d). Mesa structures are obtained as junction terminations.
A third known junction termination for a p-n junction is a device called a field plate, which is produced by applying an oxide layer to the edge area around the p-n junction and a metallic layer on the oxide layer. A field is applied to the metallic layer to change the surface potential at the edge of the p-n junction. Thus, the depletion region of the p-n junction and, therefore, the field as well can be expanded. The field plate can also be formed using an electrode layer that overlaps the oxide layer in the edge area of the p-n junction, which is provided for the p-n junction for the application of an operating voltage. A junction termination can also be formed by combining field plates and field rings (xe2x80x9cModern Power Devices,xe2x80x9d p. 119).
In a fourth known embodiment of a junction termination, ion implantation is used to introduce opposite charges in a controlled manner in the surface of the silicon layer provided as a drift region. Such a junction termination is called a xe2x80x9cjunction termination extensionxe2x80x9d. The implanted region is of the same conduction type as the diffused semiconductor region of the p-n junction, and, thus, it not only is doped with the opposite charge compared to the drift region, but it also has a lower doping amount than the diffused region. In this fourth embodiment, in addition to a region diffused into the drift region, the p-n junction can also be formed using a silicon layer arranged on the surface of the drift region with opposite doping in relation to the drift region. Ion implantation into the junction termination then takes place at the edge of the two silicon layers forming the p-n junction. The p-n junction is virtually extended by this xe2x80x9cjunction termination extension,xe2x80x9d the electric field is broadened, and the field curvature is reduced. The breakdown strength of the component is therefore increased.
Another junction termination comparable with the junction termination extension is described in Swiss Patent No. A-659,542 and referred to there as a barrier layer extension area. This junction termination is provided for a p-n junction as a bipolar active semiconductor area of a semiconductor component and can be produced by ion implantation or epitaxial growth. The lateral dimension (WJER) of the barrier layer extension area is set greater than approximately one-half of the depletion width (Wid) of the low-dope side of the p-n junction. For lateral dimensions of more than one-half of the depletion width (Wid) , no further improvement is obtained in this prior art junction termination.
In xe2x80x9cModern Power Devices,xe2x80x9d p. 128, the xe2x80x9cjunction termination extensionxe2x80x9d is proposed for bipolar transistors (BJT), field-effect transistors (MOSFETs), and thyristors (silicon-controlled rectifiers or SCRs). Due to the additional parasitic diode created with this junction termination, however, bipolar leak currents are generated in the off state of the component and high stored charges during the operation of the component, which can result in serious problems, especially in the case of a unipolar silicon MOSFET. These leak currents and stored charges increase considerably if the junction termination is enlarged, since the charge carrier injection of the parasitic diode increases with the surface area of the junction termination.
An object of the present invention is to provide a silicon-based semiconductor component with a junction termination that does not considerably increase the stored charge in the on state of the component.
This object is achieved in accordance with a semiconductor component comprising at least one semiconductor region made of silicon of a first conduction type which acquires a depletion region in the active area of the component when an off-state voltage is applied to the active area. A junction termination for the active area is formed by silicon of the opposite conduction type compared to the semiconductor region taking on a depletion region. The junction termination is arranged around the active area in or on the surface of this semiconductor region. A doping agent (dopant) having a low impurity energy level of at least 0.1 eV (100 meV) is provided for this junction termination. For a junction termination with p-type conduction, the dopant is an acceptor and its energy level is an acceptor level, given with reference to the valence band of the silicon crystal. For a junction termination with n-type conduction, the dopant is a donor, and its energy level is a donor level, given with reference to the conduction band of the silicon crystal.
The operation of the semiconductor component of the present invention is based on the fact that the dopant atoms (impurity atoms, atomic lattice defects) in the junction termination are virtually non-ionized at relatively low on-state voltages, which typically is up to a maximum of 5 volts in the on state of the semiconductor component, within the temperature range allowed for silicon. This is due to the low energy levels of the dopant atoms. On the other hand, the dopant atoms are at least mostly ionized at the high off-state voltages of typically 100 volts to 5000 volts in the off state of the semiconductor component. In the on state of the semiconductor component, the voltage drop across the active area remains below that of the parasitic p-n diode formed with the junction termination and the semiconductor area with opposite doping, and the junction termination emits basically no charge carrier. Thus, virtually no additional stored charges are produced by the junction termination. This is a considerable advantage in comparison with the prior art silicon junction terminations doped with dopants having relatively flat energy levels such as boron (B), with an acceptor level of 0.045 eV, phosphorus (P), with a donor level of 0.045 eV, or arsenic (As), with a donor level of 0.054 eV. In contrast, considerably higher electric fields are applied to the junction termination in the off state of the semiconductor element due to the relatively high off-state voltages. The deep-lying impurities in the silicon of the junction termination are at least mostly ionized by these electric fields and produce a stable space charge according to their spatial distribution. This space charge also electrically shields the active area of the semiconductor component against external fields and charges.
Other acceptors for the junction termination include beryllium (Be), with an acceptor level of 0.17 eV, zinc (Zn), with an acceptor level of 0.26 eV, nickel (Ni), with an acceptor level of 0.23 eV, cobalt (Co), with an acceptor level of 0.35 eV, magnesium (Mg), with an acceptor level of 0.17 eV, tin (Sn), with an acceptor level of 0.27 eV, and/or indium (In), with an acceptor level of 0.16 eV. Sulfur (S), with a donor level of 0.26 eV, selenium (Se), with a donor level of 0.25 eV, and/or titanium (Ti), with a donor level of 0.21 eV are preferably used as donors.
The junction termination can be configured in the form of a field ring structure.
In a further embodiment of the present semiconductor component, the silicon semiconductor region is expanded, preferably in at least one lateral direction, to take on the depletion region of the active area, for example, as a silicon epitaxial layer, and the vertical dimension of the depletion region depends on the off-state voltage applied to the active area. At least one lateral dimension of the junction termination is now greater than the maximum vertical dimension of the depletion region (maximum depletion region depth). A lateral direction is defined as a direction that is basically parallel to a surface of the silicon semiconductor area, and a vertical direction is defined as a direction basically perpendicular to the surface of the semiconductor area. Therefore, a relatively large-surface p-n junction is built into the semiconductor component between the junction termination and the semiconductor region. The electric field is widened and, similar to the active area of the semiconductor component, almost fully shielded against external charges and fields in the area of the drift region surface due to the removal of the charge carriers from the space charge zone of this built-in p-n junction. An avalanche breakdown takes place far from the surface of the semiconductor region, at a certain depth. Because the junction termination has a greater lateral dimension than the maximum vertical dimension of the depletion region, the breakdown voltage of the semiconductor component is considerably less sensitive to doping or to charge carrier concentration fluctuations in the junction termination. In this embodiment, the junction termination is preferably directly adjacent to the active area.
The depletion region of the p-n junction is formed by the junction termination and the semiconductor region. This depletion region, and thus, the widening of the electric field on the surface of the semiconductor region, can be further adjusted by setting the lateral dimension or the vertical dimension or the doping profile of the junction termination. Such an adjustment further increases the breakdown strength and the adjustment tolerance of the semiconductor component.
The lateral dimension of the junction termination is preferably set to be greater than the maximum vertical dimension of the space charge zone of the semiconductor area by a factor of three.
In a further embodiment, the junction termination includes at least two semiconductor areas of different dopings. In this embodiment, smoother widening of the electric field can be achieved. The semiconductor component is especially robust with regard to manufacturing tolerances with such a multistage-doped junction termination. The at least two semiconductor areas can be arranged vertically on top of one another or laterally next to one another.
In another embodiment, an electric contact provided for the active area of the semiconductor component can also at least partially overlap the junction termination. Thus, the junction termination can be set to a pre-defined potential. The junction termination can be grown epitaxially on the surface of the semiconductor area provided as a drift region or produced by diffusion or ion implantation into the semiconductor area.
The semiconductor component preferably has at least one unipolar active area, for example, a MISFET structure or a Schottky diode structure, but can also be provided with at least one bipolar active area such as, for example, a p-n diode, an IGBT, GTO or thyristor structure.